Percussion drill bits are widely used both for drilling relatively shallow bores in hard rock and for creating deep boreholes. For the latter application, a drill string is typically used in which a plurality of rods are coupled end-to-end via threaded joints as the depth of the hole increases. A terrestrial machine is operative to transfer a combined impact and rotary drive motion to an upper end of the drill string whilst a drill bit positioned at the lower end is operative to crush the rock and form the boreholes. WO 2006/033606 discloses a typical drill bit comprising a drill head that mounts a plurality of hard cutting inserts, commonly referred to as buttons. Such buttons comprise a carbide based material to enhance the lifetime of the drill bit.
Fluid is typically flushed through the drill string and exits at the base of the borehole via apertures in the drill head to flush the rock cuttings from the boring region to be conveyed rearward through the bore around the outside of the drill string. Further examples of percussive drill bits are disclosed in U.S. Pat. No. 3,388,756; GB 692,373; RU 2019674; U.S. 2002/0153174; U.S. Pat. No. 3,357,507, U.S. 2008/0087473; U.S. Pat. No. 4,113,037; GB 2011286; U.S. Pat. No. 5,890,551; DE 2856205 and WO 2009/067073.
The effectiveness of the drill bit to bore into rock is dependent on the rocks breaking resistance that may be considered to include vertical and horizontal stresses imposed to the rock within the subterranean depth. Drill head design and construction is typically a compromise between maximising the drill bit operational lifetime and maximising the axially forward cutting performance. The drill bit must also facilitate rearward transport of the rock fragments within the borehole that would otherwise decrease forward cutting. Accordingly, what is required is a drill bit and in particular a bit head that is optimised to satisfy the above considerations.